High strength cold rolled steel sheet for automotive use

ABSTRACT

A high strength cold rolled steel sheet has a composition consisting of the following elements (in wt. %): C 0.07-0.15, Mn 2.3-3.2, Si 0.6-1.2, Cr 0.05-0.5, Al≤0.2 Nb≤0.1, balance Fe apart from impurities, a multiphase microstructure comprising a matrix of bainitic ferrite, and a tensile strength (R m ) of 980-1100 MPa.

TECHNICAL FIELD

The present invention relates to high strength steel sheets suitable forapplications in automobiles. In particular, the invention relates tocold rolled steel sheets having a tensile strength of at least 980 MPaand an excellent formability.

BACKGROUND ART

For a great variety of applications increased strength levels are apre-requisite for light-weight constructions in particular in theautomotive industry, since car body mass reduction results in reducedfuel consumption.

Automotive body parts are often stamped out of sheet steels, formingcomplex structural members of thin sheet. However, such parts cannot beproduced from conventional high strength steels, because of a too lowformability for complex structural parts. For this reason multi phaseTransformation Induced Plasticity aided steels (TRIP steels) have gainedconsiderable interest in the last years, in particular for use in autobody structural parts and as seat frame materials.

TRIP steels possess a multi-phase microstructure, which includes ameta-stable retained austenite phase, which is capable of producing theTRIP effect. When the steel is deformed, the austenite transforms intomartensite, which results in remarkable work hardening. This hardeningeffect, acts to resist necking in the material and postpone failure insheet forming operations. The microstructure of a TRIP steel can greatlyalter its mechanical properties. The most important aspects of the TRIPsteel microstructure are the volume percentage, size and morphology ofthe retained austenite phase, as these properties directly affect theaustenite to martensite transformation, when the steel is deformed.There are several ways by which it is possible to chemically stabilizeaustenite at room temperature. In low alloy TRIP steels the austenite isstabilized through its carbon content and the small size of theaustenite grains. The carbon content necessary to stabilize austenite isapproximately 1 wt. %. However, high carbon content in steel cannot beused in many applications because of impaired weldability.

Specific processing routs are therefore required to concentrate thecarbon into the austenite in order to stabilize it at room temperature.A common TRIP steel chemistry also contains small additions of otherelements to help in stabilizing the austenite as well as to aid in thecreation of microstructures which partition carbon into the austenite.In order to inhibit the austenite to decompose during the bainitetransformation it has generally been considered necessary that thesilicon content should be about 1.5 wt. %. The most common alloyingaddition is 1.5 wt. % of both Si and Mn.

TRIP-aided steel with a Bainitic Ferrite matrix (TBF)-steels have beenknown for long and attracted a lot of interest, mainly because thebainitic ferrite matrix allows an excellent stretch flangability.Moreover, the TRIP effect ensured by the strain-induced transformationof metastable retained austenite islands into martensite, remarkablyimproves their drawability.

The formability of TRIP steels is heavily affected by the transformationcharacteristics of the retained austenite phase, which is in turnaffected by the austenite chemistry, its morphology and other factors.In ISIJ International Vol. 50(2010), No. 1, p. 162-168 aspectsinfluencing the formability of TBF steels having a tensile strength ofat least 980 MPa are discussed. However, the cold rolled materialsexamined in this document were annealed at 950° C. and austempered at300-500° C. for 200 s in salt bath. Accordingly, due to the highannealing temperature these materials are not suited for the productionin a conventional industrial annealing line.

However, the high Si-contents generally used in TBF-steels result in theformation of silicon oxide layers on the surface of the steel strip,which may adhere to the rolls in the continuous annealing line (CAL) andgive rise to surface defects on subsequently produced steel sheets.Therefore, in recent years it has been a strive to reduce the siliconcontent in TBF steels.

WO20131144377 discloses a cold rolled TBF-steel sheet alloyed with Siand Al and having a tensile strength of at least 980 MPa. WO2013/144376discloses a cold rolled TBF-steel sheet alloyed with Si and Cr and atensile strength of at least 980 MPa. Although these steels discloseseveral attractive properties there is demand for 980 MPa steel sheetshaving an improved property profile with respect to advanced formingoperations, where both local elongation and total elongation is ofimportance such as for structural members in automobile seats.

DISCLOSURE OF THE INVENTION

The present invention is directed to high strength (TBF) steel sheetshaving a tensile strength of 980-1100 MPa and an excellent formability,wherein it should be possible to produce the steel sheets on anindustrial scale in a Continuous Annealing Line (CAL). The inventionaims at providing a steel composition that can be processed tocomplicated structural members, where both local elongation and totalelongation is of importance, in particular for automobile seatcomponents. However, it is generally considered, that if the totalelongation is increased, then the properties governed by the localelongation such as the hole expanding ratio (HER) or (λ) isdeteriorated.

DETAILED DESCRIPTION

The invention is described in the claims.

The steel sheet has a composition consisting of the following alloyingelements (in wt.

C 0.07-0.15 Mn 2.3-3.2 Si 0.6-1.2 Cr 0.05-0.5  Al ≤0.2 Mb ≤0.1

the balance consists of iron and impurities.

The importance of the separate elements and their interaction with eachother as well as the limitations of the chemical ingredients of theclaimed alloy are briefly explained in the following. All percentagesfor the chemical composition of the steel are given in weight % (wt. %)throughout the description. The amount of hard phases is given in volume% (vol. %). Upper and lower limits of the individual elements can befreely combined within the limits set out in the claims.

C: 0.07-0.15%

C stabilizes the austenite and is important for obtaining sufficientcarbon within the retained austenite phase. C is also important forobtaining the desired strength level. Generally, an increase of thetensile strength in the order of 100 MPa per 0.1% C can be expected.When C is lower than 0.07% then it is difficult to attain a tensilestrength of 980 MPa. If C exceeds 0.15%, then the weldability isimpaired. The upper limit may be 0.14, 0.13 or 0.12%. The lower limitmay be 0.08, 0.09, or 0.10%. A preferred range is 0.08-0.13%.

Mn: 2.3-3.2%

Manganese is a solid solution strengthening element, which stabilisesthe austenite by lowering the M, temperature and prevents ferrite andpearlite to be formed during cooling. In addition, Mn lowers the A_(c3)temperature and is important for the austenite stability. At a contentof less than 2.3% it might be difficult to obtain the desired amount ofretained austenite, a tensile strength of 980 MPa and the austenitizingtemperature might be too high for conventional industrial annealinglines. In addition, at lower contents it may be difficult to avoid theformation of polygonal ferrite. However, if the amount of Mn is higherthan 3.2%, problems with segregation may occur because Mn accumulates inthe liquid phase and causes banding resulting in a potentiallydeteriorated workability. The upper limit may therefore be 3.1, 3.0,2.9, 2.8 or 2.7%. The lower limit may be 2.3, 2.4, or 2.5%.

Si: 0.6-1.2%

Si acts as a solid solution strengthening element and is important forsecuring the strength of the thin steel sheet. Si suppresses thecementite precipitation and is essential for austenite stabilization.

However, if the content is too high, then to much silicon oxides willform on the strip surface, which may lead to cladding on the rolls inthe CAL and surface defects on subsequently produced steel sheets. Theupper limit is therefore 1.2% and may be restricted to 1.1, 1.05, 1.0 or0.95%. The lower limit may be 0.65, 0.7, 0.75 or 0.80%. A preferredrange is 0.7-1.0%.

Cr: 0.05-0.5%

Cr is effective in increasing the strength of the steel sheet. Cr is anelement that forms ferrite and retards the formation of pearlite andbainite. The A_(c3) temperature and the M % temperature are onlyslightly lowered with increasing Cr content. Cr results in an increasedamount of stabilized retained austenite. The amount of Cr is limited to0.7%. The upper limit may be 0.65, 0.60, 0.55, 0.50, 0.45 or 0.40, 0.35,0.30 or 0.25%. The lower limit may be 0.10, or 0.15%. A preferred rangeis 0.1-0.3%.

Si+Cr: 0.9-1.3%

It is preferred that the amount of Si+Cr is in the range of 0.9-1.3%because when added in combination Si and Cr have a synergistic effectand result in an increased amount of retained austenite, which, in turn,results in an improved ductility. For these reasons the amount of Si+Cris preferably limited to the range of 0.9 to 1.2%.

Al: ≤0.2%

Al promotes ferrite formation and is also commonly used as a deoxidizer.The M, temperature is increased with an increasing Al content. A furtherdrawback of Al is that it results in a drastic increase in the A_(c3)temperature and therefore makes it more difficult to austenitize thesteel in the CAL. For these reasons the Al content is preferably limitedto less than 0.1%, more preferably to less than 0.08%. It is thuspreferred to only use Al for deoxidation. The upper level may then be0.09, 0.08, 0.07 or 0.06%. For securing a certain effect the lower levelmay set to 0.005, 0.01, 0.02 or 0.03%.

Nb: <0.1%

Nb is commonly used in low alloyed steels for improving strength andtoughness, because of its influence on the grain size. Nb increases thestrength elongation balance by refining the matrix microstructure andthe retained austenite phase due to precipitation of NbC. The steel maycontain Nb in an amount of ≤0.05%, preferably ≤0.03%. A deliberateaddition of Nb is not necessary according to the present invention. Theupper limit may therefore be restricted to ≤0.01%.

The high strength TRIP-assisted bainitic ferrite (TBF) steel sheets ofthe present invention have microstructure mainly consisting of retainedaustenite inclusions embedded in the matrix.

The microstructural constituents are in the following expressed involume % (vol. %).

The steel comprises a matrix of bainitic ferrite (BF). Hence, the amountof bainitic ferrite is generally ≥50% and may be ≥55%, ≥60% or ≥65%. Themicrostructure may also contain tempered martensite (TM). Theconstituents BF and TM may be difficult to distinguish from each other.Therefore, the total content of both constituents may be limited to70-90%. The amount is normally in the range of 80-90%.

Martensitic may be present in the final microstructure because,depending on its stability, some austenite may transform to martensiteduring cooling at the end of the overaging step. Martensite may bepresent in an amount of ≤15%. The amount of un-tempered martensite ispreferably limited to 10, 9, 8, 7, 6 or 5%. These un-tempered martensiteparticles are often in close contact with the retained austeniteparticles and they are therefore often referred to asmartensite-austenite (MA) particles.

Retained austenite is a prerequisite for obtaining the desired TRIPeffect. The amount of retained austenite should therefore be in therange of 2-20%, preferably 5-15%. The amount of retained austenite wasmeasured by means of the saturation magnetization method described indetail in Proc. Int. Conf. on TRIP-aided high strength ferrous alloys(2002), Ghent, Belgium, p. 61-64.

Polygonal ferrite (PF) is not a desired microstructural constituent andis therefore limited to ≤10%, preferably ≤5%, ≤3% or ≤1%. Mostpreferably, the steel is free from PF.

The mechanical properties of the claimed steel are important and atleast one of the following requirements should be fulfilled:

tensile strength (R_(m)) 980-1100 MPa yield strength (R_(p0.2))  580-920MPa total elongation (A₅₀) ≥13% hole expansion ratio (λ) ≥50% yieldratio (R_(p0.2)/R_(m)) ≥0.75

Preferably, all these requirements are fulfilled at the same time.

The R_(m). R_(p0.2) values are derived according to the European norm EN10002 Part 1, wherein the samples were taken in the longitudinaldirection of the strip. The total elongation (A₅₀) is derived inaccordance with the Japanese Industrial Standard JIS Z 2241: 2011,wherein the samples are taken in the transversal direction of the strip.

The mechanical properties of the steel sheets of the present inventioncan be largely adjusted by the alloying composition and themicrostructure. The microstructure may be adjusted by the heat treatmentin the CAL, in particular by the isothermal treatment temperature in theoveraging step.

According to one aspect of the invention there is provided a highstrength cold rolled steel sheet having

-   -   a) a composition consisting of the following elements (in wt.        %):

C 0.07-0.15 Mn 2.3-3.2 Si 0.6-1.2 Cr 0.05-0.5  Al ≤0.2 Nb ≤0.1

-   -   -   balance Fe apart from impurities,

    -   b) a multiphase microstructure comprising a matrix of bainitic        ferrite,

    -   c) a tensile strength (R_(m)) of 980-1100 MPa

According to another aspect of the invention there is provided a highstrength cold rolled steel sheet, fulfilling at least one of thefollowing requirements

-   -   a) a composition fulfilling at least one of the following        requirements (in wt. %):

C 0.08-0.14 Mn 2.4-3.1 Si 0.7-1.1 Cr 0.05-0.45 Al 0.005-0.1  Mb ≤0.05

-   -   -   wherein the impurities fulfil at least one of the            requirements:

Ti ≤0.05 Mo ≤0.05 N ≤0.015 B ≤0.005

-   -   -   balance Fe apart from impurities.

    -   b) a multiphase microstructure comprising at least one of (in        vol. %):

retained austenite 2-20 martensite ≤15 bainitic ferrite ≥50 polygonalferrite ≤10

-   -   c) at least one of the following mechanical properties

tensile strength (R_(m)) 990-1100 MPa yield strength (R_(p0.2))  580-920MPa total elongation (A₅₀) ≥13% hole expansion ratio (λ) ≥50% yieldratio (R_(p0.2)/R_(m)) ≥0.75

According to another aspect of the invention there is provided a highstrength cold rolled steel sheet having:

-   -   a) a composition fulfilling at least one of the following        requirements (in wt. %):

C 0.08-0.13 Mn 2.5-3.0 Si 0.75-1.05 Cr 0.1-0.4 Si + Cr 0.9-1.3 Al0.01-0.08 Mb ≤0.01

-   -   -   wherein the impurities fulfil at least one of the            requirements:

Ti ≤0.02 V ≤0.02 Mo ≤0.03 N ≤0.008 B ≤0.003

-   -   -   balance Fe apart from impurities,

    -   b) a multiphase microstructure comprising (in vol. %)

retained austenite 5-15 martensite ≤10 bainitic ferrite ≥60 polygonalferrite ≤5

-   -   c) at least one of the following mechanical properties

a tensile strength (R_(m)) 1000-1100 MPa a yield strength (R_(p0.2)) 750-900 MPa a hole expansion ratio ≥60% a yield ratio (R_(p0.2)/R_(m))0.76-0.85

According to another aspect of the invention there is provided a highstrength cold rolled steel sheet fulfilling at least one of thefollowing requirements:

-   -   a) a composition fulfilling at least one of the following        requirements (in wt. %):

C 0.09-0.12 Mn 2.5-2.9 Si 0.75-1.0  Cr 0.1-0.3 Si + Cr 0.9-1.2 Al0.01-0.05

-   -   -   wherein the impurities fulfil at least one of the            requirements:

Ti ≤0.01 V ≤0.02 Mo ≤0.03 N ≤0.008 B ≤0.003balance Fe apart from impurities,

-   -   c) at least one of the following mechanical properties

a tensile strength (R_(m)) ≥1020 MPa a yield strength (R_(p0.2))  ≥800MPa a yield ratio (R_(p0.2)/R_(m)) ≥0.78

According to another aspect of the invention there is provided a highstrength cold rolled steel sheet fulfilling the following requirements:

-   -   a) a composition consisting of (in wt. %):

C 0.08-0.14 Mn 2.4-3.1 Si 0.7-1.1 Cr 0.05-0.45 Al 0.005-0.1  Nb ≤0.05

-   -   -   wherein the impurities fulfil the requirements:

Ti ≤0.05 Mo ≤0.05 N ≤0.015 B ≤0.005

-   -   -   balance Fe apart from impurities,        -   and/or

    -   b) a multiphase microstructure comprising (in vol. %):

retained austenite 2-20 martensite ≤15 bainitic ferrite ≥50 polygonalferrite ≤10

-   -   -   and/or

    -   c) the following mechanical properties

tensile strength (R_(m)) 1000-1100 MPa yield strength (R_(p0.2)) 580-920 MPa total elongation (A₅₀) ≥13% hole expansion ratio (λ) ≥50%yield ratio (R_(p0.2)/R_(m)) ≤0.84

According to another aspect of the invention there is provided a highstrength cold rolled steel sheet fulfilling the following requirements:

-   -   a) a composition fulfilling the following requirements (in wt.        %):

C 0.08-0.13 Mn 2.5-3.0 Si 0.7-1.1 Cr 0.1-0.4 Si + Cr 0.9-1.3 Al0.01-0.08

-   -   -   wherein the impurities fulfil the requirements:

Ti ≤0.02 V ≤0.02 Mo ≤0.03 N ≤0.008 B ≤0.003

-   -   -   balance Fe apart from impurities,        -   and/or

    -   b) a multiphase microstructure comprising (in vol. %)

retained austenite 5-15 martensite ≤10 bainitic ferrite ≥60 polygonalferrite ≤5

-   -   -   and/or

    -   c) the following mechanical properties

a tensile strength (R_(m)) 1000-1100 MPa a yield strength (R_(p0.2)) 750-900 MPa a hole expansion ratio ≥60% a yield ratio (R_(p0.2)/R_(m))0.78-0.83

According to another aspect of the invention there is provided a highstrength cold rolled steel sheet fulfilling the following requirements:

-   -   a) a composition fulfilling the following requirements (in wt.        %):

C 0.09-0.12 Mn 2.5-2.9 Si 0.7-1.1 Cr 0.1-0.3 Si + Cr 0.9-1.2 Al0.01-0.05

-   -   -   wherein the impurities fulfil the requirements:

Ti ≤0.01 V ≤0.02 Mo ≤0.03 N ≤0.008 B ≤0.003

-   -   -   balance Fe apart from impurities,        -   and/or

    -   c) the following mechanical properties

tensile strength (R_(m)) 1000-1100 MPa yield strength (R_(p0.2)) 750-920 MPa total elongation (A₅₀) ≥13% hole expansion ratio (λ) ≥50%yield ratio (R_(p0.2)/R_(m)) 0.78-0.82

According to another aspect of the invention there is provided a highstrength cold rolled steel sheet as defined above, wherein the thicknessof the cold rolled sheet is 1.0-1.6 mm, preferably 1.1-1.5 mm, morepreferably 1.2-1.4 mm.

According to another aspect of the invention there is provided a highstrength cold rolled steel sheet as defined above, wherein the totalcontent of bainitic ferrite and tempered martensite is 70-90 vol. %,preferably 80-90 vol. %.

According to another aspect of the invention there is provided a highstrength cold rolled steel sheet as defined above, wherein the productof the tensile strength (R_(m)) and the total elongation (A₅₀) is ≥13000MPa %, preferably ≥13500 MPa %.

EXAMPLES

Table 1 disclose the composition of the examined steel sheets.

TABLE 1 Composition of examined steel sheets. Example C Si Mn Cr Al Inv.1 0.105 0.81 2.63 0.195 0.045 lnv. 2 0.106 0.84 2.67 0.197 0.048 Inv. 30.106 0.84 2.67 0.197 0.048 Inv. 4 0.105 0.81 2.63 0.195 0.045 Inv. 50.118 0.94 2.77 0.17 0.051

Heats of the steel alloys were produced in a continuous caster. Theslabs were reheated and subjected to hot rolling to a thickness of about2.8 mm. The hot rolling finishing temperature was about 900° C. and thecoiling temperature about 550° C. The hot rolled strips were pickled andbatch annealed at about 625° C. for a time of 10 hours in order toreduce the tensile strength of the hot rolled strip and thereby reducingthe cold rolling forces. The strips were thereafter cold rolled in afive stand cold rolling mill to a final thickness of about 1.4 mm andfinally subjected to continuous annealing.

Table 2 discloses the hot and cold rolling parameters. The batchannealing was performed between the hot- and cold rolling steps forabout 10 h.

TABLE 2 Hot and cold rolling parameters. Hot rolled Batch annealing Coldrolling Cold rolling thickness temperature thickness reduction Example(mm) (° C.) (mm) (%) Inv. 1 2.80 623 1.41 50 Inv. 2 2.79 623 1.41 49Inv. 3 2.78 625 1.41 49 Inv. 4 2.79 623 1.41 49 lnv. 5 2.79 624 1.42 49

The annealing cycle consisted of heating to a temperature of about 850°C., soaking for about 120 s, slow gas jet cooling at a rate of about 10°C./s to a temperature of about 750° C., rapid gas cooling at a rate ofabout 40° C./s to an overaging temperature of about 390-400° C.,isothermal holding at the overaging temperature and final cooling toambient temperature. The details of the treatment in the CAL are givenin Table 3.

TABLE 3 Parameters of the treatment in the CAL. Annealing Slow Jet RapidJet temp. Cooling temp. Cooling temp. Example (° C.) (° C.) (° C.) Inv.1 850 750 393 Inv. 2 850 750 397 Inv. 3 846 750 397 Inv. 4 842 750 394Inv. 5 847 750 391

The material produced according to the invention was found to haveexcellent mechanical properties as shown in Table 4. All examples had amatrix of bainitic ferrite and contained less than 10% martensite andminimal amounts of ferrite.

In particular, it may be noted that all inventive examples disclose atotal elongation (A₅₀) of more than 13% at the same time as the holeexpansibility (λ), as measured by the hole expansion test, exceeded 52%for all inventive examples.

TABLE 4 Mechanical properties. Yield Tensile Total Elongation, HoleStrength Strength A₅₀ expanding R_(p0.2) R_(m) Yield ratio (transversal)ratio λ Example (MPa) (MPa) (R_(p0.2)/R_(m)) (%) (%) Inv. 1 838 10380.81 13.4 53.6 Inv. 2 806 1018 0.79 13.2 64 Inv. 3 841 1038 0.81 14.271.8 Inv. 4 817 1027 0.80 13.4 67.6 Inv. 5 863 1084 0.80 13.5 52.2

The R_(m) and R_(p0). values are derived according to the European normEN 10002 Part 1, wherein the samples were taken in the longitudinaldirection of the strip. The elongation (A₅₀) is derived in accordancewith the Japanese Industrial Standard JIS Z 2241: 2011 for samples takenin the transversal direction of the strip.

The hole expanding ratio (λ) is reported as the mean value of threesamples subjected to hole expansion tests (HET). It was determined bythe hole expanding test method according to ISO/TS16630:2009 (E). Inthis test a conical punch having an apex of 60° is forced into a 10 mmdiameter punched hole made in a steel sheet having the size of 100×100mm². The test is stopped as soon as the first crack is determined andthe hole diameter is measured in two directions orthogonal to eachother. The arithmetic mean value is used for the calculation.

The hole expanding ratio (λ) in % is calculated as follows:

λ=(Dh−Do)/Do×100

wherein Do is the diameter of the hole at the beginning (10 mm) and Dhis the diameter of the hole after the test.

INDUSTRIAL APPLICABILITY

The material of the present invention can be widely applied to highstrength structural parts in automobiles. The high strength steel sheetsare particularly well suited for the production of parts having highdemands on the total elongation and at the same time a low edge cracksensitivity.

1. A high strength cold rolled steel sheet having; a) a compositionconsisting of the following elements (in wt. %) C 0.07-0.13 Mn 2.3-3.1Si 0.65-1.2  Cr 0.05-0.5  Al ≤0.2  Nb ≤0.05

balance Fe apart from impurities, b) a multiphase microstructurecomprising a matrix of bainitic ferrite and ≤10 volume % polygonalferrite, and c) a tensile strength (R_(m)) of 980-1100 MPa.
 2. The highstrength cold rolled steel sheet according to claim 1, fulfilling atleast one of the following requirements: a) a composition fulfilling atleast one of the following requirements (in wt. %) C 0.08-0.13 Mn2.4-3.1 Si 0.7-1.1 Cr 0.05-0.45 Al 0.005-0.1  Nb ≤0.05

wherein the impurities fulfil at least one of the requirements Ti ≤0.05Mo ≤0.05 N ≤0.015 B ≤0.05

balance Fe apart from impurities, b) a multiphase microstructurecomprising at least one of (in vol. %) retained austenite 2-20martensite ≤15 bainitic ferrite ≥50 polygonal ferrite  ≤10,

and c) at least one of the following mechanical properties tensilestrength (R_(m)) 990-1100 MPa yield strength (R_(p0.2))  580-920 MPatotal elongation (A₅₀) ≥13% hole expansion ratio (λ) ≥50% yield ratio(R_(p0.2)/R_(m)) ≥0.75.


3. The high strength cold rolled steel sheet according to claim 1,having: a) a composition fulfilling at least one of the followingrequirements (in wt. %) C 0.08-0.13 Mn 2.5-3.0 Si 0.75-1.05 Cr 0.1-0.4Si + Cr 0.9-1.3 Al 0.01-0.08 Nb ≤0.01

wherein the impurities fulfil at least one of the requirements Ti ≤0.02V ≤0.02 Mo ≤0.03 N ≤0.008 B ≤0.003

balance Fe apart from impurities, b) a multiphase microstructurecomprising (in vol. %) retained austenite 5-15 martensite ≤10 bainiticferrite ≥60 polygonal ferrite   ≤5,

and c) at least one of the following mechanical properties tensilestrength (R_(m)) 1000-1100 MPa yield strength (R_(p0.2))  750-900 MPahole expansion ratio (λ) ≥60% yield ratio (R_(p0.2)/R_(m)) 0.76-0.85.


4. The high strength cold rolled steel sheet according to claim 1,fulfilling at least one of the following requirements: a) a compositionfulfilling at least one of the following requirements (in wt. %) C0.09-0.12 Mn 2.5-2.9 Si 0.75-1.0  Cr 0.1-0.3 Si + Cr 0.9-1.2 Al0.01-0.05

wherein the impurities fulfil at least one of the requirements Ti ≤0.01V ≤0.02 Mo ≤0.03 N ≤0.008 B ≤0.003

balance Fe apart from impurities, and b) at least one of the followingmechanical properties tensile strength (R_(m)) ≥1020 MPa yield strength(R_(p0.2))  ≥800 MPa yield ratio (R_(p0.2)/R_(m)) ≥0.78.


5. The high strength cold rolled steel sheet according to claim 1,fulfilling the following requirements: a) a composition consisting of(in wt. %) C 0.08-0.13 Mn 2.4-3.1 Si 0.7-1.1 Cr 0.05-0.45 Al 0.005-0.1 Nb ≤0.05

wherein the impurities fulfil the requirements Ti ≤0.05 Mo ≤0.05 N≤0.015 B ≤0.005

balance Fe apart from impurities, and/or b) a multiphase microstructurecomprising (in vol. %) retained austenite 2-20 martensite ≤15 bainiticferrite ≥50 polygonal ferrite ≤10.

and/or c) the following mechanical properties tensile strength (R_(m))1000-1100 MPa yield strength (R_(p0.2))  580-920 MPa total elongation(A₅₀) ≥13% hole expansion ratio (λ) ≥50% yield ratio (R_(p0.2)/R_(m))≥0.84.


6. The high strength cold rolled steel sheet according to claim 3,fulfilling the following requirements: a) a composition fulfilling thefollowing requirements (in wt. %) C 0.08-0.13 Mn 2.5-3.0 Si 0.7-1.1 Cr0.1-0.4 Si + Cr 0.9-1.3 Al 0.01-0.08

wherein the impurities fulfil the requirements Ti ≤0.02 V ≤0.02 Mo ≤0.03N ≤0.008 B ≤0.003

balance Fe apart from impurities, and/or b) a multiphase microstructurecomprising (in vol. %) retained austenite 5-15 martensite ≤10 bainiticferrite ≥60 polygonal ferrite ≤5,

and/or c) the following mechanical properties tensile strength (R_(m))1000-1100 MPa yield strength (R_(p0.2))  750-900 MPa hole expansionratio (λ) ≥60% yield ratio (R_(p0.2)/R_(m)) 0.78-0.83.


7. The high strength cold rolled steel sheet according to claim 1,fulfilling the following requirements: a) a composition fulfilling thefollowing requirements (in wt. %) C 0.09-0.12 Mn 2.5-2.9 Si 0.7-1.1 Cr0.1-0.3 Si + Cr 0.9-1.2 Al 0.01-0.05

wherein the impurities fulfil the requirements Ti ≤0.01 V ≤0.02 Mo ≤0.03N ≤0.008 B ≤0.003

balance Fe apart from impurities, and/or b) the following mechanicalproperties tensile strength (R_(m)) 1000-1100 MPa yield strength(R_(p0.2))  750-920 MPa total elongation (A₅₀) ≥13% hole expansion ratio(λ) ≥50% yield ratio (R_(p0.2)/R_(m)) 0.78-0.82.


8. The high strength cold rolled steel sheet according to claim 1,wherein a thickness of the cold rolled sheet is 1.0-1.6 mm.
 9. The highstrength cold rolled steel sheet according to claim 1, wherein a totalcontent of bainitic ferrite and tempered martensite is 70-90 vol. %. 10.The high strength cold rolled steel sheet according to claim 1, whereina product of the tensile strength (R_(m)) and a total elongation (A₅₀)is ≥13000 MPa %.